CF40B Repaintings

A chessboard $ n×m $ in size is given. During the zero minute we repaint all the black squares to the 0 color. During the $ i $ -th minute we repaint to the $ i $ color the initially black squares that have exactly four corner-adjacent squares painted $ i-1 $ (all such squares are repainted simultaneously). This process continues ad infinitum. You have to figure out how many squares we repainted exactly $ x $ times. The upper left square of the board has to be assumed to be always black. Two squares are called corner-adjacent, if they have exactly one common point.

The first line contains integers $ n $ and $ m $ ( $ 1

Print how many squares will be painted exactly $ x $ times.